1. Field of the Invention
The present invention relates to a cathode ray tube (CRT) display apparatus, or more specifically, to a CRT display apparatus having a driving IC to control the high voltage output of a fly back transformer (FBT)
2. Description of the Related Art
A CRT display apparatus is an apparatus to display a picture by displaying a plurality of pixels on a screen using electric charges which are discharged from a cathode ray tube and impact a fluorescent plate.
To discharge and control the electric charges in the cathode ray tube, an extremely high voltage must be applied. The high voltage can be acquired by using an induction coil having a high turn ratio. A module comprising a plurality of such induction coils is called an FBT. When the input source voltage is about 140-200V, the high voltage induced in the secondary induction coil is about 26 kV.
Generally, a high voltage driving IC is used on an input part of the FBT to drive the source voltage.
The high voltage driving IC applies a pulse width modulation (PWM) signal into a primary induction coil of the FBT. The high voltage driving IC also controls a duty ratio to acquire a required output from the FBT if a step-up circuit is in use. The high voltage driving IC is provided with a series of rectangular pulses as a synchronization signal or as a control signal to decide the duty ratio of an output waveform. The high voltage driving IC may use a PWM control IC examples of which include models such as, UC3842 and TL494CN.
However, if a direct current signal is applied to the high voltage driving IC due to abnormal operation of the system, a conventional high voltage driving circuit may have a problem resulting in unstably driving the high voltage driving IC and a circuit component of an output part connected to an output terminal. Accordingly, if a direct current signal is applied to the high voltage driving IC over a predetermined period of time, a circuit component connected to the output terminal such as a transistor may be damaged due to an abnormal output signal from the high voltage driving IC.